The present invention relates to a method for forming carbon nanotube films, carbon nanotube films obtained by using the foregoing method, and a field emission electron source using the carbon nanotube films.
In comparison with an electron source using a thermal energy (a thermal electron emission source), a field emission electron source, because of its longer life, is often used as a power saving energy source. The field emission electron source is made of, for example, a tungsten, a silicon, a molybdenum, etc.
The field emission electron source must be provided with a sharp tip for it is at the tip at which an electric field is focused. However, it is difficult to sharpen the tip of a metallic material such as tungsten. In addition, in order for the field emission electron source to maintain the tip sharpness, it is necessary to keep an interior of an electron tube under a high vacuum of more than 10xe2x88x928 Torr. That is, it is difficult not only to manufacture such a field emission electron source made of a metallic material but also an electron tube therefor.
Recently, as a material to overcome the above-mentioned shortcomings, carbon nanotubes have been investigated. Since the carbon nanotube is chemically stable, has a sufficient mechanical stiffness, and has an enough sharpness to focus the electric field, it is ideally suited to be used as a field emission electron source.
The carbon nanotube is classified into a multi-walled carbon nanotube and a single-walled carbon nanotube. As known from their names, the multi-walled carbon nanotube is composed of two or more walled concentric cylinders, and the single-walled nanotube is composed of one walled cylinders. The multi-walled carbon nanotube has a closed tip and the single-walled carbon nanotube has an open tip. Of the two, the multi-walled carbon nanotube is mainly used as the field emission electron source.
The multi-walled carbon nanotube can be obtained by a DC arc-discharge using a pair of pure carbon electrodes under a gas atmosphere. To be more specific, the arc-discharge evaporates a positive carbon electrode to form soots and negative deposits, the negative deposits incorporating therein the multi-walled carbon nanotubes.
It has been disclosed in P. G. Collins et al., Appl. Phys. Lett69(13)23, Sep. (1996)., p1969 that, in order to use the multi-walled carbon nanotube as the field emission electron source, the obtained multi-walled carbon nanotube must be solidified by an epoxy resin without undergoing a separating and a refining processes.
However, it has been disclosed in Smally et al., Science vol.269, 1550(1995) that the multi-walled carbon nanotube with its tip opened by the separating and the refining processes is suitable for use as the field emission electron source since it has a low threshold and a large electric current density. The separating and the refining processes are carried out as follows: first coarse multi-walled carbon nanotubes are pulverized using a mortar, are dispersed in ethanol, and are sonicated; the ethanol in which the coarse multi-walled carbon nanotubes are dispersed is filtered and then is dried; the dried materials are passed through a sieve; and the sieved materials are heated and combusted on a quartz glass using a burner.
The multi-walled carbon nanotube separated and refined in the above-mentioned manner, since it has a high purity and an opened tip, is suitable for use as the field emission electron source.
In order to use the multi-walled carbon nanotube as the field emission electron source, it is preferable that the multi-walled carbon nanotube is a film form. The method for forming a multi-walled carbon nanotube film is disclosed in Walt A. de Heer, et al., Science 268(1995)845. To be more specific, after filtering the separated and the refined multi-walled carbon nanotube, it is passed through a ceramic filter having a plurality of openings, each of the openings having 0.2 xcexcm diameter, and then is deposited on a Teflon or an aluminum foil. Further, there is disclosed in Science 270(1995) 1179 the experimental analysis of the field emission electron using the multi-walled carbon nanotube film obtained by the foregoing method.
In addition to the separating and the refining processes being cumbersome, the recovery ratio of the multi-walled carbon nanotube to the consumption of the raw material is not high enough making the field emission electron source using the multi-walled carbon nanotube expensive and unsuitable for mass production.
In addition, the method for forming the carbon nanotube film according to Walt A. de Heer, et al. is not suitable for the single-walled carbon nanotube. To be more specific, since, unlike the multi-walled carbon nanotube, the single-walled carbon nanotube is flexible and gets bundled easily, it does not render itself to be captured by openings of the ceramic filter and is difficult to be deposited.
Further, in order to form the field emission electron sources for various uses, it is necessary to pattern the carbon nanotube into predetermined shapes in film form. This, however, is extremely difficult.
It is, therefore, a primary object of the present invention to provide a method for carbon nanotube films whereby the usual separating and the refining processes are eliminated, enabling a field emission electron source to be provided at a lower cost.
It is an another object of the present invention to provide a method suitable for forming a single-walled carbon nanotube film.
In accordance with one aspect of the present invention, there is provided a method for forming a carbon nanotube film, the method comprising the steps of: dispersing a solution having a solvent into which a coarse carbon nanotube is dispersed, evaporating the solvent, disposing a substrate in the solution, the substrate having an exposed portion patterned into a predetermined shape, and depositing a carbon nanotube on the exposed portion of the substrate.
In accordance with another aspect of the present invention, there is provided a method for forming a carbon nanotube film, the method comprising the steps of: preparing a solution having a solvent into which a coarse carbon nanotube is dispersed, preparing a substrate having an exposed portion patterned into a predetermined shape, scattering the solution to the exposed portion of the substrate using a scattering means, and depositing a carbon nanotube on the exposed portion of the substrate by evaporating the solvent.
In accordance with further another aspect of the present invention, there is provided a carbon nanotube filmed by using the foregoing methods.
In accordance with further another aspect of the present invention, there is provided a field emission electron source made of a carbon nanotube filmed by using the foregoing methods.